This invention relates to heavy, multi-axle, self-propelled traction vehicles known as locomotives, and it relates more particularly to an improved method of adjusting the distribution of the total static weight of a locomotive so that a predetermined distribution of axle loads is consistently obtained.
A locomotive typically comprises a body supported near its opposite ends on a pair of truck assemblies or bogies. The body includes a main frame or platform, a superstructure, and various systems, subsystems, apparatus and components that are located in the superstructure or attached to the platform. Each truck assembly includes a frame and two or more axle-wheel sets supporting the frame by means of journals near opposite ends of each axle. In addition, in a truck assembly of the kind herein contemplated, a "floating bolster" or centerplate is disposed between the truck frame and a cooperating load-transmitting pin on the underside of the platform.
In modern practice, each locomotive truck also includes two or more electric traction motors, one per axle-wheel set. Each motor is hung on an axle inboard with respect to the associated wheels, and its rotor is mechanically coupled via torque amplifying gearing to that axle. A 3-axle truck can be of either symmetrical or asymmetrical construction. If the center axle were located midway between the other two, the truck would be symmetric; if not, it would be asymmetric.
The electric current for the axle-mounted traction motors can be derived from a wayside source of electric power, or it can be generated on board the locomotive by a dynamoelectric machine driven by a prime mover that is mounted on the platform in the locomotive body. Usually the prime mover is a diesel engine, in which case the locomotive is popularly referred to as a diesel-electric locomotive. Locomotives weighing more than 300,000 pounds with engines rated more than 3,500 horsepower are common today.
Some locomotive users require that the total weight of each unit in a fleet of similar locomotives be distributed substantially equally on all of its axles when weighed at rest. When a locomotive is built in a normal manner, its static weight is not likely to be equally distributed among all of its axles, and the pattern of axle load distribution can vary appreciably from one locomotive to another of the same model. The weight on one (or more) axle might exceed a specified maximum axle load limit, while the weights on other axles of the same locomotive might be below a specified minimum load limit.
In order to control the axle load distribution, a special shimming procedure has heretofore been developed. After a diesel-electric locomotive has been built and diesel fuel, lube oil and cooling water have been added, the wheels of the fully equipped and serviced locomotive are individually weighed on a scale comprising two load cells for each axle of a truck to determine the actual load distribution among the respective axles of the locomotive. If the load distribution is not as desired, it is adjusted by selectively adding relatively thin metal spacers or shims to both the primary and the secondary suspension systems of the locomotive.
The primary suspension system of a locomotive comprises a plurality of conventional helical springs disposed in compression between spring seats on the respective axle journal boxes and cooperating pockets in the frame of each of the two locomotive trucks. The secondary suspension system typically comprises a plurality of rubber "bolster mounts" that provide controlled lateral motion. As is shown in U.S. Pat. No. 2,907,282-Erzer, such bolster mounts are disposed in compression between each truck frame and load points at four different corners of the cooperating floating bolster which in turn centrally supports one end of the locomotive body. Adding a shim between an axle spring and its spring seat, or between a bolster mount and its load point, will enable that elastic member to transmit more load (weight) for a given overall deflection, whereby a larger share of the total weight will shift to the corresponding load point on the axle or on the truck frame, as the case may be.
The sizes of the shims that are required at the respective axle springs and bolster mounts to obtain a predetermined axle load distribution could be determined for each locomotive by trial and error, but this would be very time consuming and expensive. Any increment in weight caused by an added shim at one point must of course result in a net weight decrement of equal amount at other load points, since the total weight of the locomotive is constant. On a locomotive having three axles per truck, there are six separate load points (two per axle) in the primary suspension system of each truck and eight separate load points (four per truck) in the secondary suspension system of the locomotive. Consequently the relationship between shims sizes and deviations from desired weight distribution is statically indeterminate.
Nevertheless, for each different locomotive model it is possible to prepare, from experimental measurements, tables or charts that show how variations of shim sizes at each different load point will influence the distribution of weight at all load points, and to combine such data in a useful matrix that correlates measured deviations between actual and desired weights at the respective load points to combinations of primary and secondary shims that will reduce the deviations to within acceptable limits. Such a matrix has been developed and is embodied in a computer program to predict the particular combination and sizes of shims that are required to obtain a substantially equal distribution of load on all six axles of a diesel-electric locomotive having three axles per truck. In practice, this program is executed twice. The first time it responds to the weight distribution data as measured by the six load cells of the above-mentioned scale to determine the required bolster shims. After these shims (if any) have been added to the secondary suspension, the locomotive is weighed again to re-measure the loads on its individual wheels, the corrected weight distribution data is entered in the computer, the program is run a second time, and the predicted axle shims are then added as a final adjustment to balance the axle loads.
There are two drawbacks in the foregoing method for equalizing the weight distribution of a locomotive. The shims that are added are not permanently attached, and they do not remain in place when a locomotive is "untrucked" (i.e., when its two trucks are removed from the locomotive body) for maintenance or repair purposes. Furthermore, if either truck were replaced by a different one, or if both trucks were replaced by a different pair, or if the axles were changed, the whole locomotive would need to be rebalanced. The interaction between locomotive body and trucks affects the weight distribution among the six axles, and therefore if the No. 1 truck were interchanged with the No. 2 truck the original shim set would no longer be suitable for the new combination of body and trucks. Consequently, trucks cannot be interchanged without reshimming.
A scale with separate load cells for the respective wheels of a locomotive truck has heretofore been used to check for equal weight distribution among the axles of the truck in a test rig wherein four rams are lowered in the same horizontal plane and with equal forces onto the respective load pads of the frame of an individual truck so as to match the load that will be impressed thereon when the locomotive body is placed on a pair of trucks. This checking method would be reasonably accurate if the truck were symmetrical with a center of gravity located substantially half way between its front and rear axles and if the weight of the locomotive body were divided equally among the load pads of the two trucks in service. But, for reasons explained hereinafter, the known method is not satisfactory for verifying equal axle loading in a locomotive having 3-axle asymmetrical trucks with floating bolsters.